The present invention relates to an air conditioning system that controls an operating current to within the maximum allowable range and that has performance control operation for the compressor in accordance with the air conditioning load.
FIG. 1 is an outline diagram showing the configuration of a control portion of a conventional air conditioning system of this type. In the figure, to a commercial AC power source 1 is connected an inverter 4 via a rectifier circuit 2 and a smoothing capacitor 3, and the variable frequency AC output of the inverter 4 is supplied to a compressor motor 5. In addition, to the commercial power source is also connected a DC power circuit 6 that becomes the power source for equipment that includes a central processing unit (hereinafter termed CPU) 11 comprising a microcomputer, for example. Both outputs of a temperature sensor 7 comprising thermistors and a current transformer 9 is supplied to the CPU II. The transformer 9 detects the size of an AC current which is supplied from the commercial power source 1 to the rectifier circuit 2.
The CPU 11 comprises an analog/digital (A/D) converter 12 for functionally reading both the output of the sensor 7 and transformer 9 as digital data, a frequency determining means 13 for determining an output frequency of the inverter 4 in accordance with the temperature detected by the sensor 7 and indicating the output frequency to the inverter 4, and a current limit means 14 for applying a limit to the output frequency indicated to the inverter 4 by the frequency determining means 13 in the manner that the value of the commercial current supplied to the inverter 4 from the commercial power does not exceed an allowable value.
The current limit means 14 restricts a current capacity of the air conditioning system to less than a predetermined value. Accordingly, the output frequency of the inverter 4 is usually determined in accordance with the temperature detected by the sensor 7. However, if the current supplied to the compressor motor from the AC power source becomes large and exceeds the predetermined value, the output frequency of the inverter 4 is lowered and the current to the air conditioning system is also lowered.
Namely, according to an increase in the air conditioning load, the value of the DC current supplied from the power source 1 is increased. However, many general households do not have the construction which is capable of supplying the sufficient DC current through the power sockets and electrical wirings so that the current capacity of the system of an air conditioning apparatus is limited to about 20 A, for example. For this reason, the system of the inverter that consumes a large current is provided with the current transformer 9 and the current detection signals are applied to the CPU 11 via a second input circuit 10. These analog current signals applied to the CPU II are converted into digital current data by the A/D converter 12. The current limit means 14 compares the allowable current data stored in the ROM (not indicated in the figure) with the current data converted by the A/D converter 12 and places a limit on the output frequency of the frequency determining means 13 so that the current supplied from the commercial power source 1 does not exceed the allowable value. Furthermore, the limit of the frequency is attained by performing control to lower the frequency each time the operating time exceeds a predetermined value for a high zone of a plural number of zones into which the current control range has been divided but the description of this frequency control is omitted as it has already been proposed.
A following comparison type of A/D converter is used for the A/D converter 12 that comprises the conventional air conditioning system as has been described above, and 0-5 V analog input signals corresponding to a current of 0-25 A detected of 5.0 V of the DC current circuit 6 as the reference voltage, is converted into 8-bit digital data and output. Accordingly, if the current supplied to the current transformer 9 changes by 0.1 A, then the output digital data changes by only "1".
In this case, if a high-accuracy apparatus is used as DC current circuit 6, then there is a precise correspondence between the output data of the A/D converter 12 and the actual current value detected by the current transformer 9. However, in this type of air conditioning system, there occur discrepancies in the output voltage due to a scattering of the characteristics of the circuit parts for which temporal changes have been sufficiently suppressed, and the relationship between the actual current data and the data output from the A/D converter 12 changes.
FIG. 2 is a diagram describing this relationship. Here, a current of 18A is flowing in the current transformer 9. In this status, if the control power voltage which is the output of the DC current circuit 6, is 0.5 V, then the data output from the A/D converter 12 becomes "180". Here, suppressing the width of scattering of the control power voltage to 0.2 V and applying a control power voltage of 5.2 V to the A/D converter 12 causes the data output from the A/D converter 12 to become "177" and if a control power voltage of 4.8 V is applied to the A/D converter 12, the data output from the A/D converter 12 becomes "183". In other words, the scattering of the output voltage of the D/C current circuit 6 actually changes the 18A current signals to 17.7 A or to 18.3 A.
In the same manner, when an actual current of 19 A is flowing in the current transformer 9, it is changed to 18.7 A or to 19.3 A.
In this manner, when the control power current is 4.8 V, the current data is larger than the actual value by 0.3 A and predicting this so that the current flowing to the inverter 4 is limited to 19.5 A, means that the allowable current data in the ROM referred to by the inverter 4 must be set to "192" corresponding to 19.2 A. Accordingly, in the status where the data in the ROM is set to "192", if the control voltage is 5.2 V, the actual current is limited to 18.9 A and it is no longer possible to have the maximum air conditioning performance.
The above description has been done for the changing of the value for the current detected by the current transformer 9 but the voltage that is impressed to the temperature sensor 7 is supplied from the DC current circuit 6 and this voltage is proportional to the control power voltage so that the A/D converter 12 does not change the temperature data.
When the value for the current supplied from the commercial power source 1 to a conventional air conditioning system is limited to less than an allowable value, then there is the problem that there must be operation for which the performance is suppressed.